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Bottle Radio 



Make] Projects 

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build, hack, tweak, share, discover,- 



Bottle Radio 



Written By: Steve Hobley 



TOOLS: 



PARTS: 



Drill (1) 

Pliers (1) 

from RadioShack. 

Screwdriver (1) 

Wire cutters (1) 
from RadioShack. 



Magnet wire (1) 
from RadioShack. 

150pf ceramic capacitor (1) 
from RadioShack. 

47kQ Resistor (1) 
from RadioShack. 

Speaker Wire (antenna) (1) 
from RadioShack. 

Mini Amplifier (1) 
from RadioShack. 

Hookup wire (1) 
from RadioShack. 



NTE 109 Germanium Diode (1) 
from RadioShack. 

Crystal earphone (1) 

glass bottle (empty) (1) 

150-grit sandpaper (1) 

wood, various (1) 

coat hanger wire (1) 

copper ground spike (1) 

#8 Insulated Ring Terminal (1) 
from RadioShack. 

screws and washers (1) 
Lacquer (1) 
3.5mm audio cable (1) 
Clear tape (1) 

PVCTpnpMl 



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Bottle Radio 



from RadioShack. 



SUMMARY 



Crystal radio technology has been around for many years. This "bottle radio" take on a 
crystal radio requires no power source, operates on the power from radio waves, and 
receives signal from a long wire antenna. As radio stations slowly move away from the AM 
band, the "window of opportunity" to experience this remarkable technology is dwindling. On 
that note, the closer you are to an AM station or transmitter, the more likely you are to hear 
distinct radio waves with this project. 

The "crystal" in question is contained inside a germanium diode, and is used to rectify the 
radio signal so that our ears can hear it. Since this project has no power source, the signal 
can be quite weak - crystal earphones are a fun way to hear low-amp signal (due to their 
sensitivity), or, alternatively, you could hook up a 9V mini-amplifier. 

For more information on this battery-less technology check out this Wikipedia article . 



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Bottle Radio 



Step 1 — Winding the coil. 




• Unsurprisingly, the first thing we'll need is a bottle. I'm using a brown glass bottle. Remove 
the label and rinse it clean. 

• Next, we need one of our three spools of magnet wire. Here we'll be using the red 30- 
gauge wire. 

• When winding a coil, it's intuitive to wind "hand over bottle," but this introduces a twist in 
the wire, something we don't want. A better way to wind a twist-less coil is to put the spool 
of wire on some kind of spindle and rotate the bottle slowly. Start by sticking down the free 
end with tape (start at the bottom of the bottle, leaving around 6" of lead wire for 
connecting later). 

• Be careful of the wire. You don't want it to "cross over" itself while winding. Try to keep a 
constant tension and apply tape as needed to save your position as you wind. You want to 
make the coils as smooth and closely packed as possible. 

• Once complete (using the whole spool) you should have something that looks like image 2 
(leave about 8-10" of lead wire at the top, for connecting later). Next we will sandpaper the 
wire, but before that, apply fast drying sprayable lacquer or shellac (if available) to hold the 
wire in position. If you don't, you risk loosening the wire when you sandpaper the wire. The 
lacquer will hold it all together. Also, don't use too much, otherwise you risk waiting over 
an hour for it to dry. 

• The last step is to take a small piece of 150 grit sandpaper, and carefully sand off the red 
coating in a vertical line (image 3), then gently sand the ends of the wire to expose the 
copper. 



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Bottle Radio 



Step 2 — Building the receiver. 




• Next we need a simple wooden base to hold our components. Scrap wood works fine for 
this project - feel free to paint or decorate your base as you please. Position the wooden 
blocks so that the two diagonal pieces (first photo, right) cradle the bottle and position the 
wiper stand (see info below) so that the wiper will touch the coil on the bottle when it's in 
the cradle. See the final set-up in the second image. Now we'll start making our 
connections! 

• In the picture I've placed 5 connection points, labelled A,B,C,D, and E. One way to 
connect the wires is to use Fahnstock clips - if you can't find any of these then screws and 
washers will work too. 

• In addition, I have created a "wiper" from some coat hanger wire that I cut and bent into 
shape. Attach some electrical insulating tape to the free end to make tuning a little easier 
(this is so you can hold and move the wiper without causing interference). 

• Place the bottle and hook up the top coil lead to A and the bottom coil lead to B. Make sure 
the wiper is in contact with the sanded-away copper track we created in the first step. 

• Later, moving the wiper up and down will effect the tuning of the coil. 

• We will now hook up our diode, capacitor, resistor and earphone as follows: The 150pF 
cap can either go across A and B, or A and the wiper (I eventually settled on between A 
and the wiper - see step 3, image 3 for detail). The 47kQ resistor goes across C and D, as 
does the earphone. 

• The germanium diode goes across D and E, with the black band (or silver, the diode's 
cathode) facing D (also, feel free to experiment with other diode types, but note that 
silicone diodes will not work with this configuration). 



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Bottle Radio 



Step 3 — Creating an antenna. 






* 



• You need two things for a crystal radio to work well. One is a good ground connection, and 
the other is a good antenna (having an AM radio station close by also helps!) 

• For the ground connection, I'm going to use the copper ground spike I installed for Tesla 
coiling - you can buy one of these from any hardware store. Alternatively you can use a 
piece of copper pipe. 

• Steve's Top Tip: Connect a hose pipe to one end of the copper pipe, turn on the 
water, and the pipe will slide into the ground with the minimum of fuss. 

• For the antenna, connect 24-gauge speaker wire to A. You should put the antenna as high 
into the air as you can - and try to get it away from any buildings. Additionally, keep it 
insulated from the ground as much as possible. Here the antenna is run up a wall - but 
putting it outside will really help. 

• The antenna will connect to A, and ground to B (I found out you could also simply touch B 
with your finger to provide ground, once the final two wires are installed). 

• Finally hook up the following: a single piece of hookup wire (black in my image) from B to 
C; and if you haven't already, a wire (red in my image) from the wiper to E. 

• You're done! You can now listen to the earphone while tuning the radio by moving the slider 
(don't forget to press B with your finger, or have a ground wire installed). The output is 
quite low, so you'll have to listen in a quiet room, or hook up the RadioShack mini amplifier 
for a bigger sound ( if you use the mini-amp, splice a 3.5mm audio cable to connections C 
& D). This will work, but if it didn't, do not despair, because next I'll show a couple of 
modifications that should help. 



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Bottle Radio 



Step 4 — Mod #1: Amplified loop antenna 






• There are two ways to detect a radio signal, either by sensing the electric field with an 
antenna or by sensing the magnetic field using a loop. 

• Cut two pieces of plywood and glue them together to make a 2' X-shape. Then wrap 14-18 
turns of wire around the outer corners as shown. This is our loop antenna. 

• If you can find a variable capacitor from an old radio, or from a radio ham store, even 
better. This will make a resonant coil. Just wire the capacitor in parallel with the loop 
antenna you made. 

• Connect the output of the loop antenna to A and B on our receiver 

• Still not enough signal? OK, let's build an amplifier! 



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Bottle Radio 



Step 5 — Mod #2: RF amplifier. 




• This circuit uses a 9V battery to 
pull in additional signal for our 
crystal radio. Unfortunately, it 
breaks the "no battery" rule, but if 
you live in an area with low signals, 
it might be the only option for 
getting the radio to work. 

• You can find details on making this 
amplifier here . 

• Make up the circuit board using the 
etch mask as shown and connect 
the output from our resonant loop 
antenna to the input, and then 
connect the output to the A and B 
terminals on the radio. 

• Turning the 1 KQ pot will change the 
bias point of the resistor and help 
you to amplify just the frequencies 
we require. 



There are actually many ways of making a crystal radio set; this is just one of them. If you can 
find a 365pF variable capacitor, you can try making a crystal radio like this one , reputed to be an 
excellent fixed-coil radio. 



This document was last generated on 201 3-01 -30 04:51 :29 AM. 



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Page 7 of 7